1. Field of the Invention
This invention relates to a method of producing ultrathin crystalline silicon nitride on Si (111) and formation of semiconductor devices using such ultrathin crystalline silicon nitride.
2. Brief Description of the Prior Art
The continued down-scaling of the geometries in VLSI technology has involved, as a result of such down-scaling, a reduction in component film thicknesses, examples being gate dielectrics for FET semiconductor devices and the capacitor dielectric for semiconductor memory devices. Thickness uniformity requirements for such films (about 0.14 nanometers in thickness in present technology) requires extraordinary control over the silicon wafer surface morphology (i.e., subsequent interfacial roughness) to achieve necessary scaling. The acute sensitivity of interface roughness with ultrathin films is evident when one considers the control required over large (200 mm or 300 mm) wafers.
Conventional silicon semiconductor technology incorporates Si (100) substrates, largely because of interface trap density (Dit) considerations associated with oxide films on Si (100) which has been extensively researched over the past two decades. Moreover, it has been demonstrated that surface preparation methods currently developed, such as HF-last treatments, can result in a Si (100) hydrogen-terminated surface with a roughness which is unacceptable for prospective dielectric film thickness uniformity requirements.
The use of alternative dielectric materials, such as silicon nitride, has been considered as a means to increase the gate dielectric constant and also to serve as a diffusion barrier to dopants in the gate material. However, the current silicon nitride fabrication techniques on Si (100) result in an amorphous nitride or oxynitride layer which may exhibit deleterious interface states (traps) which degrade ultimate device performance.
A further problem with silicon dioxide dielectrics over Si (100) substrates is that boron from boron-doped polysilicon gate structures can diffuse through the silicon dioxide, this problem increasing with decreased gate oxide thickness geometries, thereby degrading the properties of the device, particularly in the channel region. Boron, on the other hand, does not diffuse through silicon nitride, however, the interface between silicon nitride and Si (100) results in an amorphous silicon nitride and provides an inferior structure to that with silicon dioxide by causing a disruption of the electron flow in the channel of the active semiconductor devices.
A separate problem with silicon dioxide dielectrics is that the extremely small thicknesses allow unacceptable leakage currents as a result of electrons tunneling from the gate to the drain regions of transistors. Since silicon nitride has a larger bulk dielectric constant than silicon dioxide (.about.7 compared to about 3.9), a thicker silicon nitride layer can be used which has the same capacitance density as a thinner silicon dioxide layer. Since electron tunneling currents depend exponentially on layer thickness, even an increase in dielectric thickness of about 10 to about 20 Angstroms could reduce leakage current by several orders of magnitude.